1. Field of the Invention
The present invention generally relates to a method and apparatus for generating feedback signal in a cellular mobile communication system including a plurality of base stations and, more particularly, to a feedback method and apparatus for Cooperative Multi-Point (CoMP) in which multiple base stations cooperate with each other for downlink transmission to a terminal.
2. Description of the Related Art
Mobile communication systems have evolved into high-speed, high-quality wireless packet data communication systems to provide data and multimedia services beyond the early voice-oriented services. Recently, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), and LTE-Advanced (LTE-A) defined in 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined in 3rd Generation Partnership Project-2 (3GPP2), and 802.16 defined in IEEE, have been developed to support high-speed, high-quality wireless packet data communication services.
Particularly, the LTE-A communication standard has been developed to support high speed packet data transmission and to maximize the throughput of the radio communication system with various radio access technologies. LTE-A is the evolved version of LTE to improve data transmission capability.
The existing 3rd generation wireless packet data communication systems (including HSDPA, HSUPA and HRPD) adopt Adaptive Modulation and Coding (AMC) and Channel-Sensitive Scheduling techniques to improve transmission efficiency.
In the wireless packet data communication system adopting AMC, the transmitter is capable of adjusting the data transmission amount based on channel conditions. That is, the transmitter decreases the data transmission amount for bad channel conditions so as to fix the received signal error probability at a certain level and increases the data transmission amount for good channel conditions so as to efficiently transmit a large amount of information while maintaining the received signal error probability at an intended level.
In the wireless packet data communication system adopting channel sensitive scheduling, the transmitter serves the user having a good channel condition first among a plurality of users so as to increase the system capacity as compared to allocating a channel to just one user. Such increase of system capacity is referred to as multi-user diversity gain.
When using AMC along with a Multiple Input Multiple Output (MIMO) transmission scheme, it may be necessary to take into consideration the number of spatial layers and ranks for transmitting signals. In this case, the transmitter determines the optimal data rate in consideration of the number of layers for use in MIMO transmission.
In general, OFDMA is expected to provide superior system throughput as compared to CDMA. One of the main factors that allows OFDMA to increase system throughput is the frequency domain scheduling capability. As channel sensitive scheduling increases system capacity using time-varying channel characteristics, OFDMA can be used to obtain more capacity gain using frequency-varying channel characteristics. Recently, research is being conducted to replace Code Division Multiple Access (CDMA) used in the legacy 2nd and 3rd mobile communication systems with Orthogonal Frequency Division Multiple Access (OFDMA) for the next generation mobile communication system. 3GPP and 3GPP2 are in the middle of the standardization of an OFDMA-based evolved system.
FIG. 1 is a diagram illustrating a conventional cellular mobile communication system in which the transmit/receive antenna is arranged at the center of the cells.
Referring to FIG. 1, in the cellular mobile communication system including a plurality of cells, a User Equipment (UE) receives mobile communication service from a cell selected for a semi-static duration with the above described techniques. Suppose that the cellular mobile communication system includes three cells 100, 110, and 120. Also, suppose the cell 100 serves the UEs 101 and 102 within its service area, the cell 110 serves the UE 111, and the cell 120 serves the UE 121.
The UE 102 served by the cell 100 is located far from the antenna 130 as compared to the UE 101. In this case, the UE 102 experiences significant interference from the central antenna of the neighbor cell 120 so as to be served by the UE 100 at a relatively low data rate.
When the cells 100, 110, and 120 provide the mobile communication service independently, they transmit Reference Signals (RSs) for downlink channel estimation at the recipient. Particularly in the 3GPP LTE-A system, the UE measures the channel condition between the eNB and itself using a Channel Status Information Reference Signal (CSI-RS) transmitted by the eNB.
FIG. 2 is a diagram illustrating a resource block including an CSI-RS transmitted from an eNB to a UE in a conventional LTE-A system.
Referring to FIG. 2, two CSI-RS antenna port signals are mapped to each of the positions 200 to 219. That is, the eNB transmits two CSI-RSs for downlink measurement to the UE at the position 200. When the cellular mobile communication system includes a plurality of cells as depicted in FIG. 1, the CSI-RS is transmitted in different positions corresponding to the respective cells. For example, the CSI-RS is transmitted at the position 200 for the cell 100, the position 205 for the cell 110, and the position 210 for the cell 120. The reason why the cells are allocated resources at different positions for CSI-RS transmission is to prevent the CSI-RSs of different cells from interfering among each other.
The permutation of CSI-RSs transmitted at the positions of the CSI-RS antenna ports are defined by Equation (1).
                                                        r                              l                ,                                  n                  s                                                      ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                  2                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                                              2                            ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ,                                  ⁢                  m          =          0                ,        1        ,        …        ⁢                                  ,                              N            RB                          max              ,              DL                                -          1                                    (        1        )            
In Equation (1), c denotes the pseudo-random permutation, and the initial value of the permutation generator is defined by Equation (2).cinit=210·(7·(ns+1)+l+1)·(2·NIDcell+1)+2·NIDcell+NCP  (2)
In Equation (2), l denotes an OFDM symbol order in a slot, and NCP denotes the length of a cyclic prefix (CP) used in the cell and is set to 0 or 1.
In the cellular mobile communication system depicted in FIG. 1, the UE located at the cell edge is limited in data rate due to significant interference from neighbor cells. This means that the data rate of the UE is influenced significantly by its location within the cell in the cellular mobile communication system depicted in FIG. 1. That is, although the conventional cellular mobile communication system may serve the UE located near the center of the cell at a high data rate, it is impossible to serve the UE located far from the center of the cell at a higher data rate.